Recently, a technology for digital signal processing related to moving images are remarkably progressing, and in association with this, system development aimed at realizing digital broadcasting and broadcasting-communication integration is being pursued in many countries in the world. Particularly, as service for communicating moving image data, data transfer service over the Internet using a stream distribution system is increasing. In the stream distribution system, it is general to reproduce the data received by a reception side in real time. A system using this type of system includes, for example, VOD (Video on Demand) and a streaming distribution of live images. Image distribution service over a broad and wide variety of networks such as the Internet is also rapidly developing. These image distributions are in many cases stream distribution using a compression technology such as MPEG (Moving Picture Experts Group) and H.264.
In this type of image distribution, an encoder device in a transmission side captures a moving image, compression-encodes images (hereinafter, “pictures”) composing the moving image using MPEG or H.264, stores an obtained stream in an IP (Internet Protocol) packet, and transmits the packet. Added to the stream is PTS (Presentation Time Stamp) indicating a time at which each picture in the stream is captured. The PTS corresponding to each picture is calculated by adding a maximum value of delays in encoding to STC (System Time Clock) being a reference time of the system. Therefore, when the moving image is to be reproduced from the stream, a picture corresponding to the PTS is displayed at the same timing as a time indicated by each of the PTSs, and a moving image the same as the moving image before being encoded can thereby be reproduced.
Moreover, not only images but also audio data may be multiplexed on the stream transmitted from the encoder device. The audio data is compression-encoded by a system such as MPEG-1 layer 2 or AAC (Advanced Audio Coding).
Meanwhile, a decoder device for receiving the transmitted stream uses PCR (Program Clock Reference) and SCR (System Clock Reference) contained in the received stream, to reproduce the STC in the encoder device on the transmission side. The decoder device decodes the stream to be converted into a plurality of pictures, outputs the respective pictures each at a timing at which the reproduced STC coincides with the PTS of each picture, and causes a display device such as a display to display the moving image.
Incidentally, a packet error or the like may occur over a communication line in a real-time transmission system such as the image distribution, and thus, it is quite important to provide a mechanism for detecting or correcting an error in the system. As a method of dealing with the packet error, there is a method, disclosed in Japanese Laid-open Patent Publication No. 2005-159433, of using FEC (Forward Error Correction) and ARQ (Automatic Repeat reQuest) or the like. This method is an error detection and correction method for use when real-time data such as video and audio is transmitted over an IP network by using a protocol such as RTP (Real-time Transport Protocol) standardized by, for example, IETF (Internet Engineering Task Force). In other words, this is a technology used for an error such as loss of an IP packet occurring on the communication line.
However, a stream containing a plurality of pictures is stored in the IP packet to be transmitted upon image distribution, and only the detection and the correction of the error such as the loss of the IP packet may be insufficient. More specifically, even if the IP packet is received by the decoder device on the reception side without losing it on the communication line, an error may occur in the stream stored in the IP packet. If an error occurs in the stream, then the decoder device may not reproduce a correct moving image from the stream.
Therefore, there remains a problem that appropriate reproduction of the moving image from the stream stored in the IP packet may be failed only by error determination of the IP packet.